Pros
Small but efficient and well-qualified team. High levels of independence and autonomy, meaning there's really no limit to the impact you can have on the firm. Scope for taking on more responsibility and expanding your remit is significant. Working on interesting problems in a collaborative way. Very meritocratic so if you're good this will be recognised and well-compensated. Great work-life balance - can't imagine you'd find better in this industry.
Cons
Unsuitable for people who are not self-motivated and independent - management, guidance and direction are often lacking. Poor performers get little support to improve. Flat structure and reluctance from management to expand - there is no career progression in the traditional sense (junior, senior, etc.).